An inner spiral arm in our Galaxy about 9000 l.y. from the galactic centre (i.e. at a radius of three kiloparsecs), just over a third of the way out to the Sun's orbit. It is detected by the 21-cm radiation from interstellar hydrogen gas. The three-kiloparsec arm appears to be moving outwards at a speed of about 50 km/s as well as orbiting the centre of the Galaxy.
Subjects: Astronomy and Astrophysics.